About the Opamp approah. Do I need to add bias to the SPDIF signal before feeding it to the Opamp? I mean offsetting the "bipolar" +0.25/-0.25V input signal into "single-ended" +2.75/+2.25V, before amplification.

Or should I run the Opamp with +5V single supply and simply clip the output to +5/0V?

About the Opamp approah. Do I need to add bias to the SPDIF signal before feeding it to the Opamp? I mean offsetting the "bipolar" +0.25/-0.25V input signal into "single-ended" +2.75/+2.25V, before amplification.

Or should I run the Opamp with +5V single supply and simply clip the output to +5/0V?

You can't use most op-amps because the signal is in the 6 MHz range. If the lvel is too high coming from your source, you can attenuate it with a resistor. However, I use a 74AC04 operating on a 3.3V supply to buffer the input on my unit. You don't need anything fancier. On the electrical input, I couple the S/PDIF signal through a DC blocking capacitor to the input of the 74AC04 through a 220 ohm resistor. The input is biased with two resistors to 40% of the supply voltage.

On my opto input, the output of the TORX141 is buffered through another section of the 74AC04.

A little sharing, though most of you might already know. TTL gate does not work in the case of U3 on DEM-DIR1701 instruction manual. The first gate is actually a linear amplifier and only CMOS gates are suitable for doing this.

Before I learned this, I tried 74F04 and 74LS04. No matter how I bias the input DC, these TTL gates won't have symmetric output duty cycle. And they don't amplify, instead they simply switch on or off.